Electromechanical power transfer systems for aeronautical applications may integrate main and auxiliary engine start functions with onboard electric power generating equipment. A conventional brushless, wound field synchronous machine (WFSM) is ideal for such an electromechanical power transfer system wherein it may serve as both a starter and a generator. It is a logical choice for modern variable frequency (VF) alternating current (AC) electric system architectures. A WFSM that serves as both a starter and a generator is a representative of a class of variable speed motor drives in the start mode of operation that uses a solid-state power converter to process typically high potential direct current (DC) electric power into VF AC electric power suitable for driving the variable speed AC electric machine. Typical of all variable speed synchronous motor drives, the position of the motor rotor is required to control the solid-state power converter to meet motor performance requirements.
A resolver mounted to the starter/generator rotor may provide this required rotor position information, but it is an undesirable addition because of its imposing size, weight, and complexity or unreliability penalties. Thus, it is more desirable to achieve the electric start function in a self-sensing or sensorless manner, that is, without a resolver or other overt rotor position sensing means. Additionally, the sizing of a WFSM for such a starter/generator application is for worst case starting conditions that may require a plurality of machines to start a single main engine during cold day conditions. It is thus required in some applications to parallel a multiplicity of starter/generators to provide full rated torque from each of these starter/generators at standstill.
There are many sensorless schemes to enable sensorless operation of many different classes of electric motors under a variety of different operating conditions. At rotor standstill or low speed there is insufficient back electromotive force (EMF) generated in a dynamoelectric machine to enable an accurate estimate of rotor position using only passive measurement of terminal potentials and currents. It is therefore necessary to provide some means to stimulate the machine in order to extract rotor position information.
Either the rotor or the stator may receive such stimulation. It may be either transient or continuous, and it may be of different frequencies. All known approaches require some means to stimulate the machine and some means to interpret or demodulate the stimulation response in order to provide an estimate of the rotor position. Markunas et al., herein incorporated by reference, describe one advantageous approach in U.S. Pat. No. 7,034,097.
Markunas et al. describes a carrier injection sensorless (CIS) method of estimating the position and velocity of the rotor of a WFSM. CIS works by applying a high frequency excitation signal with an electrical current or potential rotating waveform to the dynamoelectric machine at a high enough frequency that it sweeps around the stator faster than the rotor is turning, thus “viewing” the rotor from all angles. This “viewing” is possible by measuring the resulting rotating current or potential waveform, which contains information about the rotor due to rotor position dependent differences in the equivalent magnetic circuit of the dynamoelectric machine.
By transforming the rotating current waveform at the machine terminals to its stationary two-axis equivalent (αβ) and x-y plotting the result, a non-circular orbit is observable that rotates with the rotor. This is the electromagnetic (EM) image of the dynamoelectric machine and in general, each machine design has its own unique EM image. This technique works with any dynamoelectric machine that has rotor saliencies that result in a change in impedance as seen at the stator windings to the high frequency excitation signal.